config: introduce an optional event stream while parsing

config: introduce an optional event stream while parsing

This extends our config parser so that it can optionally produce an event
stream via callback function, where it reports e.g. when a comment was
parsed, or a section header, etc.

This parser will be used subsequently to handle the scenarios better where
removing config entries would make sections empty, or where a new entry
could be added to an already-existing, empty section.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>